The present invention relates to a combined traction slip- and brake slip-controlled brake system for motor vehicles comprising a brake pressure generator with an auxiliary energy supply system and a pressure compensating reservoir. An electronic measuring and control circuit determines the wheel rotation behavior and generates electrical brake pressure control signals which are adapted to be fed to electromagnetically actuatable inlet and outlet valves incorporated in the pressure fluid supply conduit to the wheel brakes and in the pressure fluid return conduit from the wheel brakes to the pressure compensating reservoir. A value arrangement is provided and by means of which pressure fluid from the auxiliary energy supply system is introduceable into the wheel brakes of the driven wheels for the purpose of controlling the traction slip.
Brake systems with slip control are known. In order to prevent a locking of the wheels due to excessive brake pressure and to avoid the dangers connected therewith, particularly the risk of skidding and the loss of steerability, the brake pressure at the controlled wheel is either reduced or held constant in the event of a wheel becoming unstable and/or a wheel-lock tendency becoming apparent. The brake pressure is increased again, if need be, after the reacceleration of the wheel. These systems have, however, no influence on the wheel-spin due to excessive drive torque.
It has also been suggested to embody and supplement brake systems of the present type such that they can also limit the traction slip. For that purpose, the slip-controlled hydraulic brake system described in German printed and patent application No. P 33 27 401.0 has been equipped with a switch valve through the intermediary of which brake pressure from an auxiliary pressure source provided for brake power boosting can be introduced directly into the wheel brake of the drive wheel having a tendency to spin the moment the traction slip gets too high. By means of this normally closed switch valve, which is only switched into its open position during the traction slip control phase and up to the point where the necessary brake pressure is reached, the brake pressure generator at the outlet of which no brake pressure is available during this control phase in which the brake is not actuated traction slip is circumvented.
According to another proposition described in German printed and published application No. P 33 38 826.1, a pedal-actuated hydraulic brake pressure generator equipped with a master cylinder is provided, and pressure is introduced from an auxiliary pressure source provided for brake and power boosting directly into the master cylinder. In this construction, the pressure is transmitted (for example, by way of multiple-way valves) to a pre-chamber and non-return valves in the cups of the master cylinder pistons, into the working chambers of the master cylinder and from there to the wheel cylinders of the driven wheels by way of the inlet valves required for the brake slip control which are normally switched so as to be open.
It is an object of the present invention to develop a brake system which is suitable to control both the brake slip and the traction slip and which is characterized by a particularly simple construction. The system can be realized by means of a slight addition to a brake slip control system requiring only minor additional expense.